The present invention relates, in general, to a mixing head, and more particularly to a mixing head for processing a reactive material mixture of at least two plastic components reacting with one another.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Mixing heads are used for processing a reactive material mixture of at least two plastic components, which react with one another. The plastic components are introduced into a mixing chamber, intimately mixed with one another in the mixing chamber, and discharged on one end of the mixing chamber, usually via a discharge pipe. The discharge is realized in most cases in a molding tool or cavity of a molding tool. To prevent plastic material from reacting out in the mixing chamber and conglutinate the latter, the mixing chamber interacts with a cleaning piston which is retracted from the mixing chamber during material discharge. When no discharge from the mixing chamber is intended after a shot cycle, the cleaning piston is moved into the mixing chamber to clean it. In the event, a new material discharge is then intended again, the cleaning piston is again retracted and the thus cleared mixing chamber can assume its intended operation. For this purpose, the cleaning piston can be operated to reversibly move back and forth. The movement is generated by means of a drive which acts on one end of the cleaning piston. The connection between drive and cleaning piston is implemented via a shaft.
At least a tolerance gap is maintained between the cleaning piston and the mixing chamber and the housing in surrounding relationship to the cleaning piston behind the mixing chamber for allowing the back-and-forth movement of the cleaning piston. Such a smallest tolerance gap depends on the used material mixture and is oftentimes sufficient to move the plastic component or the already mixed material mixture along the cleaning piston. This is referred to as a “creep” of this material “along” the cleaning piston or the attached shaft. When failing to prevent this creepage, the plastic components or the material mixture may adhere to the shaft of the cleaning piston or migrate along the cleaning piston up to the drive. This causes not only contamination in the area of the mixing head. The plastic components may also cure in the area of the drive and cause seizing of the cleaning piston altogether. In this case, the mixing head would become inoperative and would have to be repaired. Regardless, removal of material that creeps up the cleaning piston or its shaft must normally be carried out every day and is complex to implement. This poses a particular problem when low-viscosity flexible foam materials are involved which have a long reaction time. In the event, the material foam migrates into the drive, hydraulic oil is contaminated and can be cleaned again only in a very costly manner.
There is also the further problem of entrainment of the foamed material via the shaft—also called piston rod—up to the hydraulic zone (drive for the cleaning piston), thereby risking oil contamination.
It would be desirable and advantageous to provide an improved mixing head to obviate prior art shortcomings and to prevent material creepage as well as allow easy cleaning.